Mast cells have long since been recognized as potent producers of a large panel of biological highly active mediators such as biogenic amines, arachidonic acid metabolites, cytokines and chemokines, but most of their biological functions had been elusive and speculative. By taking advantage of mast cell-deficient mice, the role of mast cells in a variety of experimental settings can now be studied in detail and such approaches have dramatically altered and enlarged our knowledge about mast cell biology and function. Herein we will focus on the role of mast cells in inflammatory reactions of diverse origin such as delayed type hypersensitivity, atopy, immune complex-mediated inflammation and innate immune responses. From a current point of view, there is no doubt that the most outstanding and beneficial feature of mast cells is their recently uncovered ability to rapidly induce a life-saving inflammatory response upon encountering microbes and microbial constituents. Nevertheless, the picture is also emerging that mast cells are deeply involved in the induction and maintenance of a variety of severe allergic and autoimmune diseases. However, a deeper understanding of their activation and immune-modulatory capacity might open a new window for the development of curative strategies.